1. Field of the Invention
The present invention generally relates to a manufacturing method for a hologram. More particularly, the present invention relates to a manufacturing method for a hologram which can control the formation of a ghost hologram formed by a noise light.
2. Related Arts
When a hologram is formed by irradiating object light and reference light on to a substrate having a hologram photosensitive layer, the object light or reference light will sometimes enter off the normal optical path and consequently a ghost hologram is formed. A major cause of the formation of this ghost hologram is a surface reflected light of the object light or reference light reflected on an interface with the atmosphere both of which have a different refractive index.
As illustrated in FIG. 8, for example, there is a method of forming an interference fringe in a photosensitive layer 91a of a substrate 91 for reproduction. The reproduction method (a one-beam method using a master hologram) is as follows: The substrate 91 is disposed on a master hologram 93 with a refractive index adjusting liquid 92 interposed therebetween; a reference light 31 enters from the surface of the substrate 91; a transmitted light 311 of the reference light 31 is diffracted and reflected by the master hologram 93; and the diffracted light is interfered as an object light 32 with the reference right 31 to thereby form the interference fringe.
In this case, for example, when the transmitted light 311 transmits the master hologram 93 and is re-reflected on a surface 931, a surface reflected light (not illustrated) of the transmitted light 311 forms a ring-like interference fringe 99 illustrated in FIG. 9 in interference with the reference light 31. This constitutes a ghost hologram.
In some other cases, the object light 32 transmitted through the substrate 91 is re-reflected on a surface 911 and this surface reflected light (not illustrated) of the object light 32 forms an interference fringe in interference with the normal object light 32.
The above can also the case even when a reflection type optical element is used instead of the master hologram 93 of FIG. 8.
In the same way, also in a two-beam method in which an object light and a reference light enter from above and below a substrate having a photosensitive layer, the surface reflected lights thereof form ghost holograms on surfaces. When a hologram having a ghost hologram as above used as a master hologram is reproduced, a similar ghost hologram is formed on a reproduced hologram.
In order to control the formation of the ghost hologram due to surface reflected lights, there have been several proposals.
For example, as illustrated in FIG. 10, a method has been proposed that antireflection-coating glasses 95 should be disposed on surfaces to control the surface reflected lights (See Japanese Unexamined Patent Publication No. 4-198981).
Specifically, antireflection-coating glass 95 is disposed on surfaces of optical glasses 94 disposed on both sides of the substrate 91 adjacent the refractive index adjusting liquids 92 to control surface reflected lights 322 and 312 of the object light 32 and reference light 31 respectively.
However, in this method using the antireflection-coating glass (FIG. 10), if an incident angle .theta. to the antireflection-coating glass is large, the reflectivity increases, and as a result, a sufficient effect can not be obtained.
On the other hand, as illustrated in FIGS. 11 and 12, a method has been proposed that prisms 961 and 962 for optical path change should be disposed to prevent the surface reflected lights 312 and 322 of the reference light 31 and object light 32 respectively from entering the substrate 91. (See Japanese Unexamined Patent Publication No. 4-19898).
However, in the method in which the prisms 961 and 962 are used to prevent the surface reflected lights 312 and 322 from entering the substrate 91 again (FIGS. 11 and 12), the vertical angles .beta. and .beta. of the prisms 961 and 962 respectively are considerably large. As a result, there is a problem that the space required for manufacturing a hologram is large.